Ethylene-vinyl acetate copolymers are well known and have a variety of uses. However, because the vinyl acetate acts as a modifier to some extent, it is not possible to synthesize copolymers having at least about 35% vinyl acetate and a sufficiently low melt index (as measured by ASTM D-1238-57T) for extrusion applications such as extrusion coating. The copolymers containing greater than 35% vinyl acetate start to lose crystallinity and cannot be produced below a melt index range of 4-6 in synthesis. At a vinyl acetate content of 40%, the minimum melt index in synthesis is 9.5-11.5.
It has now been found that the melt index of ethylene-vinyl acetate copolymers having a high content of vinyl acetate can be decreased to a level sufficient for extrusion applications by mechanically working the preformed copolymers at certain temperatures and for a limited amount of time. This result is particularly surprising because it is known that mechanically working polyethylene increases the melt index, see, e.g., U.S. Pat. No. 2,956,035 and U.S. Pat. No. 3,230,288, and it has been found that mechanically working ethylene-vinyl acetate copolymers containing 0.1% of butylated hydroxy toluene, a common antioxidant, increased the melt index.